Guidance

Appendix E: Definitions of key terms

Appendix E: Definitions of key terms

Amnionicity: The number of amnions (inner membranes) that surround babies in a multiple pregnancy. Pregnancies with one amnion (so that all babies share an amniotic sac) are described as monoamniotic; pregnancies with two amnions are diamniotic; and pregnancies with three amnions are triamniotic. Also see table 1 below.

Chorionicity: The number of chorionic (outer) membranes that surround babies in a multiple pregnancy. If there is only one membrane the pregnancy is described as monochorionic; if there are two, the pregnancy is dichorionic; and if there are three, the pregnancy is trichorionic. Monochorionic twin pregnancies and dichorionic triplet pregnancies carry higher risks because babies share a placenta. Also see table 1 below.

Feto-fetal transfusion syndrome: Feto-fetal transfusion syndrome occurs when blood moves from one baby to another. The baby that loses the blood is called the donor and the baby receiving the blood is called the recipient. Feto-fetal transfusion syndrome is a complication of monochorionic multiple pregnancies arising from shared placental circulation. It is also referred to as twin-to-twin transfusion syndrome in twin pregnancies.

Specialist obstetrician: An obstetrician with a special interest, experience and knowledge of managing multiple pregnancies, and who works regularly with women with multiple pregnancies.

Zygosity: The number of fertilised eggs that result in a multiple pregnancy. If one egg is fertilised and divides into two embryos the pregnancy is described as monozygous, whereas if two eggs are fertilised and result in separate embryos the pregnancy is described as dizygous.

Table 1 Chorionicity and amnionicity

Types of twin pregnancy

Dichorionic twins: Each baby has a separate placenta.

Monochorionic diamniotic twins: Both babies share a placenta but have separate amniotic sacs.